Spamhaus CSS

What is CSS

CSS is an IP-level blocklist maintained by Spamhaus. It flags IPs that exhibit suspicious sending behavior, including:

• Snowshoe sending — Spammers distribute spam across many IPs at once, like a snowshoe spreading weight over snow, to slip past filters
• High-risk sending — IPs with patterns that look suspicious
• Compromised hosts — Devices that have been taken over and are now sending spam without the owner's knowledge

What Triggers a CSS Listing

Your IP may get listed on CSS for any of the following reasons:

• Sending unsolicited emails (recipients never opted in)
• Unusual delivery patterns that look like automated spam
• Poor email list hygiene with lots of invalid or complaint-prone addresses
• A hacked account, web form, or CMS being used to send spam
• Low overall sending reputation or signs of abuse

What Happens If You're Listed

CSS operates at the IP level. Once your sending IP is on this list, your emails are much more likely to be rejected by receiving mail servers, which can seriously hurt your deliverability.

Mitigation Process

Step 1: Investigate

Check the Spamhaus CSS website to understand why your IP was flagged.

Step 2: Fix the Problem

Before reaching out to anyone, you need to address what's causing the listing:

• Audit and clean your email list — remove invalid addresses and unengaged subscribers
• Look for any unauthorized sending activity on your account
• Scan for security vulnerabilities — close any backdoors or compromised scripts
• Review your sending practices and make sure they follow email best practices

Step 3: Submit a Delisting Request

Once you've fixed the issue, contact our Support Team with:

• The root cause that triggered the listing
• What steps you took to fix it

We'll then submit a delisting request to Spamhaus on your behalf.

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Important: Do not request delisting until the problem is actually fixed. Spamhaus will check. If they see the issue is still ongoing, they will reject the request.